1. Field of the Invention
The present invention relates to an improved catalyst composition comprising a layered catalytic material dispersed on a substrate, the catalytic material containing one or more catalytic metal components, and to a method of making the catalyst composition. More particularly, the present invention relates to a catalyst composition as aforesaid comprising an undercoat on which is dispersed an overlayer comprising at least one catalytic metal component, for example, a palladium catalytic component. The catalyst composition of the present invention is particularly well suited for the abatement of ozone and other noxious substances in an airstream.
2. Description of Related Art
U.S. Pat. No. 4,206,083 of J. C. Chang discloses the co-precipitation of platinum, palladium and manganese oxide on a ceramic support, such as a cordierite support, in order to provide a catalyst suitable for the reduction of ozone content of air intended for human respiration. The Chang Patent refers to U.S. Pat. No. 3,269,801 as evidence that it had been long recognized that ozone is present in the atmosphere in toxic concentrations at altitudes of about 9 to 46 kilometers. Aircraft flying at those altitudes scoop in cabin air from the outside atmosphere which, because it is very much compressed, is raised in temperature to several hundred degrees Centigrade. Such air is treated to reduce the ozone concentration of it to below 1 part per million ("ppm") to render it fit for use as cabin air.
U.S. Pat. No. 5,187,137 to S. Terui et al discloses an ozone abatement catalyst comprising a composition containing manganese oxide and metallic palladium and/or palladium oxide as the essential ingredients, formed as a thin film on a support. The method of preparing the catalyst includes coating on a support a slurry containing manganese oxide, metallic palladium and/or a palladium compound, and an inorganic oxide sol as a binder, for example, an alumina sol, silica sol, titania sol or zirconia sol. See column 2, line 53 through column 3, line 5 and column 4, lines 16-19. At column 3, line 62 to column 4, line 3, the Terui et al Patent discloses that the support, i.e., the substrate on which the catalytic material is disposed, may be a cordierite or other similar inorganic support, or it may be a metal support.
U.S. Pat. No. 4,900,712 of A. Bar-Ilan et al discloses a catalytic washcoat in which one or more catalytically active non-noble metal oxides ("dopants") such as iron oxide are deposited from neutral aqueous colloidal solutions thereof onto preformed high surface area crystalline alumina. The neutral colloidal compounds are said to provide a uniform, thin coating of the non-noble dopants on the alumina particles and to avoid the use of noxious elements such as nitrates or chlorides, thereby substantially eliminating any air pollution hazard. See the paragraph bridging columns 3 and 4 of the Patent, and column 5, lines 23-26. The reference to eliminating air pollution hazards appears to be with reference to the manufacture of the catalyst, see column 1, lines 29-41.
U.S. Pat. No. 4,343,776 issued to W. F. Carr discloses an ozone abatement catalyst containing at least one platinum group metal (platinum, palladium or rhodium) or catalytically active compound thereof and an oxide or aluminate of at least one non-precious Group VIII metal (iron, cobalt or nickel). By applying the non-precious metal oxide as an alumina slip prior to application of the platinum group metal component to the substrate, the platinum group metal, e.g., palladium, is preferentially exposed to the ozone. See column 2, line 53 to column 4, line 19. The carrier or support, i.e., the substrate, may be any one of a wide range of materials, including aluminum, as described at column 2, line 56 to column 3, line 3.
U.S. Pat. No. 5,250,489 to R. A. Dalla Betta et al discloses a catalyst structure configured to provide heat exchange in which the support, i.e., substrate, is a metallic support which may comprise aluminum or aluminum alloys, provided the latter are used at temperatures which will not deform or melt the material. See column 6, lines 16-22. However, other materials, including aluminum-containing steels, are preferred (column 5, line 46 et seq).
U.S. Pat. No. 5,080,882 of M. Yoshimoto et al discloses an ozone decomposition catalyst disposed on a thin porous carrier (substrate) material which has micropores of preferably not less than 30 microns in diameter for ozone abatement, in order to prevent substantial pressure loss. See column 2, lines 5-15. Any suitable known ozone catalyst may be utilized, including oxides of manganese, iron, silver, nickel or copper and a noble metal such as platinum or palladium or a mixture of two or more of these. See column 2, line 16 et seq.